1. Field of the Invention
The present invention relates to an encoder and, more particularly, to an encoder having a long length measuring stroke.
2. Related Background Art
Hitherto, an encoder having a high resolution and a long length measuring stroke has strongly been demanded.
It is sufficient to make the scale long to extend the length measuring stroke. However, it is difficult to make a long scale having a fine lattice pattern.
To avoid such a problem, the applicant of the present invention has proposed an optical type linear encoder having a high resolution and a long length measuring stroke as disclosed in JP-A-58-191906. According to the linear encoder shown in this Official Gazette, a plurality of reading heads are arranged at intervals shorter than the effective length of the lattice pattern portion formed in a scale, and a signal from each of the reading heads is selectively extracted in association with the movement of the scale thereby reading the scale. Such a type of encoder has also been disclosed in U.S. Pat. Nos. 4,158,509 and 4,096,384 and the like.
However, to switch the signals from the reading heads as mentioned above, an additional mechanism such as a sensor to monitor the position of the scale and the like is necessary, so that the construction of the encoder becomes complicated and it is not preferred.